Loss of Signalling Bearer Transport

ABSTRACT

Being aware of a loss of signalling bearer transport through an IP Connectivity Access Network is an important issue. Therefore, the present invention relies on amending the Policing and Charging Control model with means to provide the IMS infrastructure with subscriptions to and notifications about signalling session events detected on the signalling IP flow transported through the bearer layer. To this end, a P-CSCF, or AF included therein, is amended to allow the establishment of a signalling session for subscription to notification of bearer level events for a signalling IP flow. Apart from that, new processing rules are required at the AF and PCRF for handling the signalling session, the notification of events and the termination of the signalling session.

TECHNICAL FIELD

The present invention generally relates to possible loss of signallingbearer transport through a bearer network without signalling networkentities being aware of such event. In particular, the invention relatesto procedures for requesting and obtaining notifications of the losssignalling bearer transport.

BACKGROUND

The IP Multimedia Subsystem (hereinafter IMS) enables operators of aPublic Land Mobile Network (hereinafter PLMN) to provide theirsubscribers with multimedia services based and built on Internetapplications, services and protocols. Different services andapplications as defined by the 3^(rd) Generation Partnership Project(hereinafter 3GPP) can be offered on top of IMS, for example aMultimedia Telephony Service as described in 3GPP TS 22.173.

An IMS application, as well as IMS-provided services, includes aexchange of media, such as voice or video, and of signalling for a rightinterpretation and treatment of the media. In a scenario where a useraccesses the IMS through an access network based on an Internet Protocol(hereinafter IP), both signalling and media are transported through abearer layer as respective signalling and media IP packets, which aregenerally and respectively referred as signalling IP flow and media IPflows.

A complete support for IMS applications includes a user equipment usedby the user who is an IMS subscriber, an IP-Connectivity Access Network(hereinafter IP-CAN) and specific functional elements of the IMS asdescribed in 3GPP TS 23.228. An exemplary IP-CAN may be a General PacketRadio Service (hereinafter GPRS) network.

Before being authorized to make use of IMS services provided by IMSapplications, the user has to register into the IMS. However, beforebeing able to register into the IMS, the user has to get IP connectivityto IP-CAN. To this end, and in case the IP-CAN is a GPRS network, theuser has to establish a bearer for transporting signalling IP flows,namely the so-called “PDP Context”.

Even though IMS signalling and further media flows may be bothtransported in a same general purpose PDP Context, a general purpose PDPContext is preferably used as a primary dedicated PDP Context for IMSsignalling, whereas media flows are preferably transported over aseparate secondary dedicated PDP Context.

Therefore, the user accesses a Gateway GPRS Support Node (hereinafterGGSN) to establish a primary PDP Context of a general purpose or,preferably, for IMS signalling only.

Once this primary PDP Context has been established to carry out the IMSsignalling IP flow, the user can register into the IMS. The 3GPP IMSmakes use of a Session Initiation Protocol (hereinafter SIP) forcommunication between the user and IMS entities as well as between theIMS entities. The user thus makes use of a SIP Register message forregistering into the IMS. Such SIP Register is received at a Proxy CallSession Control Function (hereinafter P-CSCF) server, which is an IMSentity used for accessing the IMS.

In accordance with a well-known 3GPP IMS registration procedure, afterhaving received and processed such message, the P-CSCF forwards ittowards an Interrogating Call Session Control Function (hereinafterI-CSCF) server. The latter asks a Home Subscriber Server (hereinafterHSS) for capabilities required for assigning a Serving Call SessionControl Function (hereinafter S-CSCF) server for servicing the user.Once the I-CSCF has received such capabilities selects and assigns oneS-CSCF fulfilling these capabilities, and the original SIP Registermessage is forwarded towards the assigned S-CSCF. This S-CSCF requestsfrom the HSS a user profile for the user and, once obtained, the S-CSCFis ready for servicing the user. An IMS registration confirmation issent back towards the P-CSCF and towards the user indicating theassigned S-CSCF.

Once the user has successfully registered into the IMS, the user canaccess different IMS applications, and request different IMS services,for which different media resources have to be negotiated between theuser equipment of the originating user and either a specific IMSapplication or a user equipment of a destination user, as the IMSservice might require. To this end, the user accesses to IMS with a SIPInvite message, which is received at the P-CSCF and transmitted throughthe IMS infrastructure towards the S-CSCF and likely further towards anApplication Server (hereinafter AS) in charge of a particular IMSapplication that the user might have invoked. This message has anassociated media description required for carrying out the service. Thismedia description is sent with a Session Data Protocol (hereinafter SDP)and is used during the negotiation of the media resources required forcarrying out the service.

All the above IMS signalling, SIP signalling during the IMS registrationprocedure as well as both SIP and SDP signalling during medianegotiation procedure, are thus transported as an IMS signalling IP flowin the primary PDP Context established by the user with the GGSN.

During the execution of these IMS registration and media negotiationprocedures there is no media to be exchanged yet through the IP-CAN and,consequently, there is no IMS media IP flow through the bearer layeryet.

SUMMARY

Knowledge of a loss of signalling bearer transport through the IP-CAN isan important issue, both when the signalling bearer is used to conveysignalling for a session already established as well as when there is nosession established yet.

A loss of the IMS signalling IP flow may occur during the bearertransport through an IP-CAN connection for example due to a bad radiocondition, or to a recovery action in the GGSN or in a Serving GPRSSupport Node. Also for example, if an initial request is sent from theIMS towards a terminating user and there is a failure of the bearer thattransports the signalling, there is a number of reattempts depending ona timer before the IMS stops repeating the request unless theoriginating user clears the session. This leads to an unnecessary tyingup of resources with long waiting times for the originating user.

It is an object of the present invention to find a mechanism whereby theloss of signalling bearer transport through the IP-CAN are, at least,minimized. That is, the present invention is aimed to find a mechanismwhereby the loss of the IMS signalling IP flow is known to the IMSinfrastructure so that proper actions can be taken without wasting timeand resources.

A new architecture called Policy and Charging Control (hereinafter PCC)is currently developed under 3GPP TS 23.203 and intended to addressdifferent types of access networks. This PCC architecture canselectively control the Quality of Service (hereinafter QoS), Policingand Charging for media flows associated with IMS media sessions for IMSsubscribers.

However, this PCC functionality cannot be applied before havingestablished such IMS media sessions.

Consequently, IP flows of a primary PDP Context bearing IMS signalling,which have no associated IMS media session, are excluded from PCCcontrol on QoS requirements, Policing and Charging.

The PCC architecture includes an Application Function (hereinafter AF)offering applications that require control of IP bearer resources andlocated in the signalling plane for instance in an IMS P-CSCF; aPolicing and Charging Enforcement Function (hereinafter PCEF) located inthe traffic plane, for instance in a GGSN, and in charge of filteringmedia IP flows and installing QoS-related rules in order to applypolicing and charging per media IP flow; and a Policing and ChargingRules Function (hereinafter PCRF) interposed between the signalling andthe bearer layers and responsible for triggering the installation ofQoS-related rules towards the PCEF.

The present invention thus relies on amending the PCC model with meansto provide the IMS infrastructure with subscriptions and notificationsabout signalling session events detected on the signalling IP flowtransported through the bearer layer.

Therefore, the PCC model needs to be amended so that the AF, or the IMSP-CSCF as such, is enabled to request the establishment of an AFsignalling session, specific for signalling in the absence of mediasession information. This AF signalling session allows the AF to requestPCC control procedures, namely subscription to notification of bearerlevel events for signalling IP flows. Apart from the AF signallingsession, the PCC model needs to be amended so that new processing rulesare associated at both AF, or the P-CSCF as such, and PCRF in order tomanage this AF signalling session specific for signalling. Theseprocessing rules are in charge of the establishment of signallingsessions, notification of events, and termination of signallingsessions.

This is achieved by the teaching of the independent claims.

In accordance with a first aspect of the invention, there is provided aProxy Call Session Control Function server, hereinafter P-CSCF, foraccessing an IP Multimedia Subsystem, hereinafter IMS, via signallingtransported through a bearer layer bearing media IP flows and asignalling IP flow. This P-CSCF includes: protocol handler forregistering a subscriber into the IMS; processing rules handler forestablishing a signalling session in absence of a media session, andnotifications of signalling session events related to the signalling IPflow; and signalling session means for requesting the notification ofthe signalling session events detected on the signalling IP flow towardsa rules-control function device, hereinafter RCFD.

In this P-CSCF, the signalling session means may include: means forsubmitting towards the RCFD signalling session information related tothe signalling IP flow, in absence of media session information; andmeans for receiving the notification from the RCFD about the detectedsignalling session event. In particular, this signalling sessioninformation may include an IP address identifying a user equipment (4)used by the subscriber and IP address, and optionally a port number,identifying the P-CSCF.

Advantageously, the protocol handler of the P-CSCF may include sessiondata means for transmitting media session information between thesubscriber and the IMS, once the subscriber is registered into the IMS.

In addition to subscription and notification of signalling sessionevents, the P-CSCF may further support subscription and notification ofmedia session events. To this end, the P-CSCF may further include:Quality of Service, hereinafter QoS, rules handler for establishing amedia session including QoS-related requirements, and notifications ofmedia session events related to the media IP flows; and media sessionmeans for requesting the notification of the media session eventsdetected on the media IP flow towards the RCFD.

This P-CSCF may be implemented in such a way that the processing ruleshandler and the signalling session means, as well as the advantageousconstituent elements thereof as cited above, may be provided in anenhanced application function device based on the PCC model.

In accordance with a second aspect of the invention, there is provided aRCFD for guarantying a QoS negotiated through a signalling layer formedia transport through a bearer layer, the bearer layer being atransport layer capable of bearing media IP flows and a signalling IPflow. This RCFD comprises: a bearer session module for subscribing tobearer sessions events to be reported from an IP Connectivity AccessNetwork; processing rules handler for establishing a signalling sessionin absence of a media session; and signalling session means forreceiving a request for notification of signalling session eventsdetected on the signalling IP flow from a requester entity, and forsubmitting the notification about the detected signalling session eventtowards the requester entity. In particular, the requester entity may bethe P-CSCF or an AF device based on a PCC model and suitable for beingincluded in other network nodes participating in the signalling layer.

The request for notification received at the RCFD may include signallingsession information related to the signalling IP flow, in absence ofmedia session information. This signalling session information relatedto the signalling IP flow may include an IP address identifying thesubscriber as well as an IP address, and optionally a port number,identifying the requester entity.

Moreover, this signalling session information related to the signallingIP flow may also include an identifier of relating to a general beareror to an “IMS-only” bearer so that notifications of signalling sessionevents detected on the signalling IP flow can be received in the bearersession module as relating to a general bearer or to an “IMS-only”bearer.

In accordance with a third aspect of the invention, there is provided amethod of reporting towards an entity in a signalling layer a loss ofsignalling bearer transport occurred at a bearer layer when a useraccesses the IMS. The signalling layer offers applications that requirea QoS to be guaranteed for media transport through the bearer layer, andthe bearer layer is a transport layer capable of bearing media IP flowsand a signalling IP flow. This method comprises the steps of: forwardinga registration request received from the user at a P-CSCF to the IMSinfrastructure; establishing a signalling session in absence of a mediasession, and notifications of signalling session events related to thesignalling IP flow at the P-CSCF; requesting the notification of thesignalling session events detected on the signalling IP flow towards theRCFD; subscribing the RCFD to bearer sessions events to be reported onthe signalling IP flow from an IP Connectivity Access Network; andforwarding to the P-CSCF the signalling session events reported to theRCFD as bearer sessions events on the signalling IP flow from the IPConnectivity Access Network.

In this method, the step of requesting notification of the signallingsession events may include a step of submitting from the P-CSCF towardsthe RCFD signalling session information related to the signalling IPflow, in absence of media session information. In particular, the stepof submitting signalling session information may include a step ofsubmitting an IP address identifying the user.

Additionally, the method may further include a step of transmittingmedia session information between the subscriber and the IMS, once thesubscriber is registered into the IMS.

In this respect, the method may further include a step of establishing amedia session that includes QoS-related requirements, and notificationsof media session events related to the media IP flows at the P-CSCF; anda step of requesting the notification of the media session eventsdetected on the media IP flow towards the RCFD.

BRIEF DESCRIPTION OF THE DRAWINGS

The features, objects and advantages of the invention will becomeapparent by reading this description in conjunction with theaccompanying drawings, in which:

FIG. 1 is a block structure illustrating adaptations on a PCC model forbeing used when accessing IMS through a GPRS access network in order tobe notified about a loss of signalling bearer transport.

FIG. 2 is a basic block diagram illustrating an adapted PCC model foruse when accessing IMS through a GPRS access network.

FIGS. 3 a and 3 b show a basic sequence diagram illustrating anexemplary course of actions to be followed for carrying out a method ofreporting a loss of signalling bearer transport towards an IMS entity.

DETAILED DESCRIPTION

The following describes some preferred embodiments for adaptation of aPCC model in order to request and get a notification on the loss ofsignalling bearer transport. Even though the PCC model refers to an AF,a PCRF and a PCEF, the two latter being entities originally intended forpolicing and charging decisions and enforcement points, the presentinvention does not necessarily require such functions.

In this respect, amendments to be made on an AF 1 in accordance with thePCC model, as proposed by an aspect of the present invention, may bedirectly made on the P-CSCF 10 without substantially departing from thegeneral inventive concept behind the present invention. Moreover,amendments to be made on a PCRF 2-bis in accordance with the PCC model,as proposed by another aspect of the present invention, may be made in arules-control function device 2 (hereinafter RCFD) which is notnecessarily used for policing and charging as well.

There is thus provided a method, in accordance with the invention, forreporting towards a P-CSCF 10 located in a signalling layer a loss ofsignalling bearer transport occurred at a bearer layer when a useraccesses the IMS. The signalling layer offers applications that requirea quality of service, hereinafter QoS, to be guaranteed for mediatransport through the bearer layer, whereas the bearer layer is atransport layer capable of bearing media IP flows B-55 and a signallingIP flow B-50. In this respect, the P-CSCF is an IMS where the useraccesses the IMS through.

Prior to access the IMS, the user 4 needs to get IP connectivity throughan IP Connectivity Access Network, which in an exemplary embodimentthroughout the present application is a GPRS network. To this end, asFIG. 3 a illustrates, the user 4 requests in step S-510 theestablishment of a Primary PDP Context.

Generally speaking, a bearer-usage parameter value is used when issuingthe request to indicate how the bearer is going to be used. At present,this value may adopt one among the following values: “general” when nospecific bearer usage information is available; “IMS-only” when thebearer is going to be used only for bearing IMS signalling; and“dedicated” when the bearer is going to be used for a dedicated purposein respect of a particular media transmission. For the purpose of thepresent invention, the above request for a Primary PDP contextpreferably indicates “IMS-only” since its usage is intended for bearingIMS signalling.

Such request for establishment of a Primary PDP context is received at aGGSN 30 of a GPRS network exemplary considered the IP-CAN throughout thepresent specification. The GGSN receiving such a request sends in stepS-515 a CCR message towards a rules-control function device 2(hereinafter RCFD), which is preferably interposed between thesignalling layer and the bearer layer, and which may correspond to aPCEF 2-bis in a traditional PCC model enhanced with the novel featuresprovided for the RCFD in order to accomplish the objects of the presentinvention. Upon receipt of the CCR message, the RCFD 2 gets relevantdata about the bearer session established for this Primary PDP Context.To this end, as illustrated in FIG. 1, the RCFD 2 stores this relevantdata in a bearer session module 20 as relating to a “general” or“IMS-only” bearer in order to make a distinction versus further possiblebearers for media IP flows. This bearer session module may comprise aseparate sub-module 24 for dealing with “general” or “IMS-only” bearerfor signalling IP flows, and a separate sub-module 25 for dealing with a“dedicated” bearer for media IP flows.

Then, an acknowledge CCA message is sent in step S-520 back from theRCFD 2 towards the GGSN 30, and a corresponding PDP Context Responsemessage sent in step S-525 acknowledges back to the user 4 theestablishment of this Primary PDP Context from the GGSN 30.

Once the user has got IP connectivity, the user may access the IMS byregistering therein. To this end, the method provides for a step ofreceiving a SIP Register message in step S-530 at the P-CSCF, and a stepof forwarding the SIP Register message in step S-535 towards further IMSinfrastructure 9. In this respect, the well-known 3GPP IMS registrationprocedure, as explained above for background of the present invention,takes place. The IMS infrastructure 9 thus represents all the IMSentities referred therein, such as I-CSCF, HSS and S-CSCF, other thanthe P-CSCF. In order to carry out these steps of receiving andforwarding the SIP Register message, and as illustrated in FIG. 1, theP-CSCF is equipped with a protocol handler 16, which is in charge ofmanaging the SIP messages S-50 between the user 4 and the other IMSinfrastructure 9. In particular, this protocol handler may includededicated means 15 to deal with pure SIP messages and protocol.

At this stage, different alternative embodiments are possible toaccomplish objects of the invention. In one embodiment not drawn in anydrawing, the P-CSCF waits for the reception of a SIP “200 OK” messagefrom the IMS infrastructure indicating the successful registration ofthe user 4 in IMS before triggering the next actions. In anotherembodiment illustrated in FIG. 3 a, the P-CSCF does not wait for thesuccessful registration result and initiates the course of actionsrequired to accomplish objects of the invention. Of course, as receivingthe SIP “200 OK” message in a step S-540, such message is forwardedtowards the user 4 in a step S-565.

Therefore, irrespective on whether the SIP “200 OK” message was receivedin step S-540 or not yet, the method comprises a step of establishing asignalling session in absence of a media session during the step I-a atthe P-CSCF, as well as those notifications of signalling session eventsrelated to the signalling IP flow that the P-CSCF wants to get reported.To this end, as illustrated in FIG. 1, the P-CSCF includes processingrules handler 13 with all the logic required to create and furtheridentify such signalling sessions and such notifications.

Since there is no media session information available within the SIPRegister message, and no specific QoS or charging characteristics to beenforced for the signalling IP flows, a request for establishment of asignalling session does not require description of media components orcharging-related identifiers.

The step of establishing a signalling session in absence of a mediasession is also carried out at RCFD 2 during the step I-b. To this end,as also illustrated in FIG. 1, the RCFD 2 also includes a correspondingprocessing rules handler 23, which is connectable via interface I-13with the processing rules handler 13 of the P-CSCF, and which containsall the logic required to create and further identify the signallingsessions and notifications at the RCFD 2.

This step of establishing the signalling session in absence of a mediasession, illustrated in FIG. 3 a, is followed by a step S-545 ofrequesting the notification of the signalling session events to bedetected on the signalling IP flow B-50, by means of an AAR message,towards the RCFD 2. To this end, as illustrated in FIG. 1, the P-CSCF 10is equipped with a signalling session means 11 for requesting suchnotification of the signalling session events towards the RCFD 2.

Correspondingly and also illustrated in FIG. 1, the RCFD 2 comprisessignalling session means 21 for receiving via interface I-12 the requestfor notification of the signalling session events, and for furthersubmitting via the same interface I-12 the requested notifications.

More specifically, the step S-545 of requesting the notification of thesignalling session events may include a step of submitting viainterfaces I-12 towards the RCFD 2 signalling session informationrelated to the signalling IP flow, in absence of media sessioninformation. Where an enhanced PCEF 2-bis of a PCC model is used, theinterface I-10 is used for this submission towards the P-CSCF 10, asillustrated in FIG. 2. This signalling session information is preferablyreceived in the signalling session means 21 of the RCFD 2.

This signalling session information may basically include an indicationof “loss of signalling bearer” to be understood by the RCFD 2 as anindication that the P-CSCF 10, or the AF device 1, subscribes tosignalling bearer events to be detected on the signalling IP flow, andno specific QoS-related rules and decisions are required for thispurpose.

Advantageously, the signalling session information submitted from theP-CSCF 10 towards the RCFD 2 may include an IP address identifying thesubscriber 4. Moreover, this signalling session information mightinclude as well an IP address, and optionally a port number, identifyingthe P-CSCF 10. This signalling session information may be useful forestablishing detection criteria to appropriately detect those requiredsignalling session events related to the signalling IP flow.

The method also comprises a step of subscribing the RCFD 2 in stepS-550, by means of a RAR message, to bearer sessions events to bereported on the signalling IP flow B-50 from an entity in the IPConnectivity Access Network, which under the present embodiment may bethe GGSN 30, or a constituent element 3 included therein, such as anenhanced PCEF 2-bis in accordance with the PCC model. To this end, theRCFD 2 may associate signalling session information in the signallingsession means 21 with the bearer session sub-module 24 dealing with“general” or “IMS-only” bearer for signalling IP flows, via interfaceI-c. Then, this bearer session sub-module 24 dealing with “IMS-only”bearer for signalling IP flows is thus associated via interface I-22with a bearer control 31 module in the GGSN 30, or a constituent element3 included therein, wherein the detection of a signalling session eventfor the signalling IP flow really takes places.

Once the subscription to such signalling session events is carried outat the GGSN 30, or a constituent element 3 included therein, anacknowledgement is sent back towards the RCFD 2 in step S-555, and acorresponding acknowledge is returned towards the P-CSCF 10 in stepS-560.

If, for any reason, the Primary PDP context for IMS signalling is lost,which bearer usage is marked as “general” or “IMS-only”, the GGSN 30 orthe constituent element 3 notifies this event to the RCFD 2, likely viaa diameter CCR message, and the latter in turn notifies suchcorresponding signalling session event towards the P-CSCF 10, or towardsthe AF device 1. In this case, at the same time as reporting the eventof loss of the signalling bearer, the RCFD 2 terminates the currentsignalling session via a diameter ASR message.

For this purpose, the method also comprises a step of forwarding towardsthe P-CSCF 10 and via interface I-12 those signalling session eventsreported via interface I-22 to the RCFD 2 as bearer sessions events onthe signalling IP flow B-50, from the bearer control module 31 in anentity 3 or 30 of the IP Connectivity Access Network.

As receiving a notification of a signalling session event reporting theloss of signalling bearer, the P-CSCF 10 or the AF device 1 mayterminate the signalling session and take proper actions through thesignalling layer. To this end, the processing rules handler 13, which isalso responsible for termination of the signalling session, communicatesvia internal interface I-g with the protocol handler 16 in order totrigger the appropriate SIP messages towards the user 4 and the otherIMS infrastructure 9.

In addition, the method may include a step S-570 of receiving mediasession information at the P-CSCF 10 from the user 4, once said user isregistered into the IMS, and a step S-575 of transmitting the mediasession information towards the IMS infrastructure 9, and also viceversa. To this end, the P-CSCF includes session data means 14 in theprotocol handler 16. This session data means 14 are preferably in chargeof dealing with session data protocol (hereinafter SDP) portions usedfor transferring and negotiating media session information in bothdirections throughout the signalling layer.

The media session information may be sent in a SIP Invite message duringstep S-570, which has embedded as SDP the media session information.Such SIP Invite message is forwarded in step S-575 towards the IMSinfrastructure 9 and, from there, possibly towards a potential Actor B5-bis in step S-580, which might be an AS in charge of a particular IMSapplication, as commented above, or a destination subscriber.

Following these steps of transmitting the media session information,there may be a step S-590 of negotiating the media to be agreed on bythe parties, namely between the user equipment 4 of the originating userand the IMS infrastructure 9, or between the user equipment 4 of theoriginating user and the user equipment 5-bis of the destination user.

As FIG. 3 b illustrates, the additional method goes on with thesubmission back to the P-CSCF in steps S-610 and S-615 of informationrelated with the media agreed on for further communications. Followingthis, the method includes a step of establishing, via interface I-d inthe P-CSCF 10 and via interface I-f in the RCFD 2, a media session thatincludes QoS-related requirements. During this step, also notificationsof media session events related to the media IP flows are established atthe P-CSCF 10. To this end, the rules handler 13 of the P-CSCF is alsoin charge of establishing a media session that includes QoS-relatedrequirements, and notifications of media session events related to themedia IP flows B-55.

Aligned with the above steps of the method for notification ofsignalling session events, also for these steps related to notificationof media session events the rules handler 23 of the RCFD 2 is in chargeof establishing a media session that includes QoS-related requirementslikely in cooperation with the rules handler of the P-CSCF via interfaceI-13.

The method may further include a step S-620 of requesting thenotification of the media session events to be detected on the media IPflow B-55, from the P-CSCF 10 towards the RCFD 2. To this end, theP-CSCF includes media session means 12 for requesting the notificationof the media session events to be detected on the media IP flow B-55towards the RCFD 2, and the latter also includes media session means 22for providing the notification of the media session events to bedetected on the media IP flow B-55 towards the P-CSCF 10.

The method may further comprise a step of subscribing the RCFD 2 in stepS-625, by means of a SIP RAR message, to bearer sessions events to bereported on the media IP flow B-55 from the GGSN 30, or a constituentelement 3 included therein, such as an enhanced PCEF 2-bis in accordancewith the PCC model.

These latest additional steps of the method in order to getnotifications of media session events on the media IP flow B-55 arecompleted by corresponding acknowledgements as in previous steps S-555and S-560 respectively.

Then, upon reception of a SIP 200 OK message at the user side 4,indicating a successful negotiation of the media between the originatinguser equipment 4 and the IMS or the destination user equipment 5-bis,the user equipment may request in step S-635 the establishment of aSecondary PDP Context indicating “dedicated” as bearer usage since itsusage is intended for bearing IMS media.

The request for establishment of a Secondary PDP context is received ata GGSN 30, which sends in step S-640 a CCR message towards the RCFD 2.Upon receipt of the CCR message indicating “dedicated” for IMS media,the RCFD 2 gets relevant data about the bearer session established forthis Secondary PDP Context. To this end, as illustrated in FIG. 1, theRCFD 2 stores this relevant data in a bearer session module 20 asrelating to a “dedicated” in order to make a distinction versus theprevious bearer for a signalling IP flow. This bearer session module maycomprise a separate sub-module 25 for dealing with this “dedicated”bearer for media IP flows.

The RCFD 2 receiving the CCR message for the Second PDP Contextindicating it is a “dedicated” bearer for IMS media, installsQoS-related rules towards the GGSN 30 in step S-645, and the GGSNeventually acknowledges to the user 4 in step S-650 the establishment ofthe Secondary PDP Context.

From step S-655 on, media transmission can be exchanged in accordancewith negotiated QoS requirements.

The invention is described above in respect of several embodiments in anillustrative and non-restrictive manner. Obviously, variations, andcombinations of these embodiments are possible in light of the aboveteachings, and any modification of the embodiments that fall within thescope of the claims is intended to be included therein.

1. A Proxy Call Session Control Function server for accessing an IPMultimedia Subsystem, hereinafter IMS, via signalling transportedthrough a bearer layer bearing media IP flows and a signalling IP flow,the Proxy Call Session Control Function server comprising: protocolhandler for registering a subscriber into the IMS; processing ruleshandler for establishing a signalling session in absence of a mediasession, and notifications of signalling session events related to thesignalling IP flow; and signalling session means for requesting thenotification of the signalling session events detected on the signallingIP flow towards a rules-control function device.
 2. The Proxy CallSession Control Function server of claim 1, wherein the signallingsession means includes: means for submitting towards the rules-controlfunction device signalling session information related to the signallingIP flow, in absence of media session information; and means forreceiving the notification from the rules-control function device aboutthe detected signalling session event.
 3. The Proxy Call Session ControlFunction server of claim 2, wherein the signalling session informationrelated to the signalling IP flow includes an IP address identifying auser equipment used by the subscriber.
 4. The Proxy Call Session ControlFunction server of claim 3, wherein the signalling session informationrelated to the signalling IP flow further includes an IP address, andoptionally a port number, identifying the Proxy Call Session ControlFunction server.
 5. The Proxy Call Session Control Function server ofclaim 1, wherein the protocol handler includes session data means fortransmitting media session information between the subscriber, onceregistered, and the IMS.
 6. The Proxy Call Session Control Functionserver of claim 5, further including: Quality of Service (QoS), ruleshandler for establishing a media session that includes QoS-relatedrequirements, and notifications of media session events related to themedia IP flows; and media session means for requesting the notificationof the media session events detected on the media IP flow towards arules-control function device.
 7. (canceled)
 8. An application functiondevice for offering through a signalling layer applications that requirea quality of service (QoS), to be guaranteed for media transport througha bearer layer, the bearer layer being a transport layer capable ofbearing media IP flows and a signalling IP flow, the applicationfunction device comprising: processing rules handler for establishing asignalling session in absence of a media session, and notifications ofsignalling session events related to the signalling IP flow; andsignalling session means for requesting the notification of thesignalling session events detected on the signalling IP flow towards arules-control function device.
 9. The application function device ofclaim 8, wherein the signalling session means includes: means forsubmitting towards the rules-control function device signalling sessioninformation related to the signalling IP flow, in absence of mediasession information; and means for receiving the notification from therules-control function device about the detected signalling sessionevent.
 10. The application function device of claim 9, wherein thesignalling session information related to the signalling IP flowincludes an IP address identifying the subscriber.
 11. The applicationfunction device of claim 8, wherein the processing rules handler isadapted for terminating the signalling session.
 12. The applicationfunction device of claim 8, further comprising media session means forrequesting notifications on media session events detected on the mediaIP flows towards a rules-control function device.
 13. The applicationfunction device of claim 13, wherein the media session means includes:means for submitting towards the rules-control function device mediasession information related to the media IP flows, including QoS-relatedrequirements; and means for receiving the notification from therules-control function device about the detected media session event.14. A rules-control function device for guaranteeing a quality ofservice (QoS), negotiated through a signalling layer for media transportthrough a bearer layer, the bearer layer being a transport layer capableof bearing media IP flows and a signalling IP flow, the rules-controlfunction device comprising: a bearer session module for subscribing tobearer sessions events to be reported from an IP Connectivity AccessNetwork comprising: processing rules handler for establishing asignalling session in absence of a media session; and signalling sessionmeans for receiving a request for notification of signalling sessionevents detected on the signalling IP flow from a requester entity, andfor submitting the notification about the detected signalling sessionevent towards the requester entity.
 15. The rules-control functiondevice of claim 14, wherein the request for notification of signallingsession events includes signalling session information related to thesignalling IP flow, in absence of media session information.
 16. Therules-control function device of claim 15, wherein the signallingsession information related to the signalling IP flow includes an IPaddress identifying the subscriber.
 17. The rules-control functiondevice of claim 16, wherein the signalling session information relatedto the signalling IP flow includes an IP address, and optionally a portnumber, identifying the requester entity.
 18. The rules-control functiondevice of claim 15, wherein the signalling session information relatedto the signalling IP flow includes an identifier of relating to ageneral bearer or to an “IMS-only” bearer.
 19. The rules-controlfunction device of claim 14, wherein notifications of signalling sessionevents detected on the signalling IP flow are received in the bearersession module as relating to a general bearer or to an “IMS-only”bearer.
 20. A method of reporting towards an entity in a signallinglayer a loss of signalling bearer transport occurred at a bearer layerwhen a user accesses an IP Multimedia Subsystem, hereinafter IMS, thesignalling layer offering applications that require a quality of service(QoS), to be guaranteed for media transport through the bearer layer,the bearer layer being a transport layer capable of bearing media IPflows and a signalling IP flow, the method comprising a step of:forwarding a registration request received from the user at a Proxy CallSession Control function, hereinafter P-CSCF, towards IMSinfrastructure; the forwarding step further comprising the steps of:establishing a signalling session in absence of a media session, andnotifications of signalling session events related to the signalling IPflow at the P-CSCF; requesting the notification of the signallingsession events detected on the signalling IP flow towards arules-control function device; subscribing the rules-control functiondevice to bearer sessions events to be reported on the signalling IPflow from an IP Connectivity Access Network; and forwarding to theP-CSCF the signalling session events reported to the rules-controlfunction device as bearer sessions events on the signalling IP flow fromthe IP Connectivity Access Network.
 21. The method of claim 20, whereinthe step of requesting the notification of the signalling session eventsincludes a step of submitting from the P-CSCF towards the rules-controlfunction device signalling session information related to the signallingIP flow, in absence of media session information.
 22. The method ofclaim 21, wherein the step of submitting signalling session informationincludes a step of submitting an IP address identifying the user. 23.The method of claim 20, further including a step of transmitting mediasession information between the subscriber, once registered, and theIMS.
 24. The method of claim 23, further including the steps of:establishing a media session that includes QoS-related requirements, andnotifications of media session events related to the media IP flows atthe P-CSCF; and requesting the notification of the media session eventsdetected on the media IP flow towards a rules-control function device.